Waveguide window having circulating fluid of critical loss tangent for dampening unwanted mode



March 1969 F. o. JOHNSON 3 76 WAVEGUIDE WINDOW HAVING CIRCULATING FLUID0F CRITICAL LOSS TANGENT FOR DAMPENING UNWANTED MODE .Filed Oct. 1?,196a E INVENTOR.

r FLOYD O.JOHNSON '0' BY ATTORNEY United States Patent 4 Claims ABSTRACTOF THE DISCLOSURE A microwave waveguide window structure is disclosed.The window structure includes a dielectric Wave permeable structurehermetically sealed across the interior of a waveguide. The dielectricwindow structure includes or is constructed to define a fluid passagewaytherethrough. Means are provided for circulating a dielectric fluidthrougn the passageway, said fluid having a loss tangent between 0.001and 0.009 for mode damping of the window structure and for cooling ofthe window structure in use. By providing the loss tangent between 0.001and 0.009, the loss presented by the fluid to the transmission mode issubstantially less than that which would be presented by water as thecoolant. Therefore, a relatively small amount of the desired power to betransmitted through the window is absorbed in the dielectric fluid,thereby permitting relatively eflicient cooling of the window structurein use. However, this relatively low loss tangent provides suflicientdamping for certain resonant ghost modes to prevent sustained excitationthereof, whereby overheating of the window structure in use is preventedsince the ghost modes are effectively damped.

This invention relates, in general, to waveguide transmission, and moreparticularly to waveguide Windows, and to devices using same.

In many high frequency systems it is necessary to pass wave energythrough a barrier which separates two regions in the system, one ofwhich is normally evacuated. Ideally, such .a barrier, called a window,is vacuum tight so that the integrity of the vacuum region ismaintained, and permits passage of a broadband of frequencies at highpower, while presenting a minimum of interference so that energy willnot be reflected or dissipated as it passes through.

One major limitation in waveguide transmission has been that certainresonant modes (hereinafter referred to as resonant ghost modes) existat discrete frequency regions across the frequency band passed bywindows constructed according to the teachings of the prior art. Thehalf power points of the ghost mode bandwidth are usually 2 me. or lessapart giving a mode Q of 1000 to 2000. Such a high Q circuit, whenexcited with high RF. fields dissipates a great deal of power in thewindow in the form of heat, frequently causing the window to rupture. Inorder to prevent window failure, the frequency range of such devices hadto be limited to a range between resonant ghost modes, or the power tobe transmitted had to be decreased. Thus, a severe limitation was placedon bandwidth and power handling capabilities of devices characteristicof the prior art.

In accordance with the teachings of the present invention, the mode Q ofthe resonant ghost modes is lowered by increasing losses within thecircuit. A dielectric structure transparent to electromagnetic waves issealed within and across a waveguide. The structure has a fluidpassageway therethrough positioned in energy exchanging relationshipwith the waves that pass through. A moderately ice lossy fluid having alow dielectric constant relative to air, preferably within the range of1.0 to 3.0 inclusive is caused to circulate through the passagewaywithin the passageway through the dielectric structure. By a moderatelylossy fluid is meant one which has a loss tangent high with respect toceramic but low with respect to water, preferably within the range of0.001 and 0.009 inclusive. The use of this fluid serves both tofacilitate removal of extra heat and provide cooling of the dielectricstructure, as well as to damp out and prevent excitation of the harmfulresonant ghost modes. In this way, power handling capabilities andbandwidth are greatly increased.

It is the object of this invention, therefore, to provide a waveguidewindow, and devices using same, capable of passing high power over abroad band of frequencies.

One feature of the present invention is the provision of a microwavetransmission device including a dielectric structure sealed within awaveguide and having a fluid passageway therethrough positioned inenergy exchanging relationship with the Waves passing therethrough, andmeans for circulating a moderately lossy fluid having a low dielectricconstant relative to air through the structure.

Another feature of the present invention is the provision of a microwavetransmission device including a dielec tric structure sealed within awaveguide comprising a pair of thin spaced-apart solid dielectricmembers, the dielectric members forming with the internal walls of thewaveguide a chamber for passage of fluid therethrough, for example, amoderately lossy fluid having a low dielectric constant relative to air.

Still another feature of the present invention is the provision of amicrowave transmission device including a dielectric structure sealedwithin a waveguide comprising a half wavelength rectangular block ofdielectric material containing spaced bores within said material forpassage of a moderately lossy fluid with a low dielectric constantrelative to air therethrough.

A further feature of the present invention is the provision of amicrowave transmission device including a. dielectric structure sealedwithin a waveguide and a pair of members to act as corona shields andsupport means, disposed within the inner wall of said waveguide, one ofsaid members being juxtapositioned to either side of the dielectricstructure, the members having a curved outer surface, and preferablywith a radius of curvature greater than 0.02".

A still further feature of the present invention is the provision of amicrowave transmission device of any of the above types in an electrondischarge device.

These and other objects and features of the present invention and afurther understanding may be had by referring to the followingdescription and claims, taken in conjunction with the following drawingsin which:

FIG.1 is a plan view of an electron discharge device utilizing featuresof the present invention;

FIG. 2 is an enlarged cross-sectional view of a preferred embodiment ofthe microwave transmission device of the present invention, taken alongthe lines 22 of FIG. 1;

FIG. 3 is a cross-sectional view taken along the lines 3-3 of FIG. 2;

FIG. 4 is an enlarged cross-sectional view of another embodiment of thenovel microwave transmission device of the present invention;

FIG. 5 is a cross-sectional view taken along the lines 55 of FIG. 4;

I IG. 6 is a plot of frequency vs. voltage standing wave ratio (VSWR)for a novel waveguide window constructed in accordance with theteachings of the present i tron; and

FIG. 7 1s an enlarged cross-sectional view of the area 3 delineated bythe line 7-7 of FIG. 2.

Referring now to the drawings and in particular to FIG. 1, there isshown an electron discharge device ern ploying novel features of thepresent invention. A multicavity klystron amplifier tube 11 of the typeshown and described in more detail in US. application Ser. No. 148,-520, filed Oct. 30, 1961 and assigned to the same assignee as thepresent invention, comprises three main portions: a beam producingsection 12 on one end which serves to form and project a beam ofelectrons over a predetermined path directed axially and longitudinallyof the tube 11; a central beam interaction section 13 where interactiontakes place between the projected electron beam and an appliedelectromagnetic wave to produce amplification of the wave; and, acollector structure 14 at the terminating end of the tube 11 where theelectrons of the spent beam are collected. A coolant such as water issupplied to the collector structure 14 via fluid fittings 15 andcirculates through ducts (not shown) in the collector structure 14.

The tube 11 is evacuated to a suitable low pressure, for example, 10-torr. Input wave energy to be amplified is coupled to the upstream endof the beam interaction section 13 via the intermediary of a rectangularwaveguide 16 and through a vacuum tight Waveguide structure 17 whichsupports a window sealed therein (not shown) transparent toelectromagnetic waves. Amplified output wave energy is extracted in theconventional manner at the downstream end of the beam interactionsection 13 via the intermediary of a rectangular waveguide 18 andthrough an output waveguide window structure 19 to be described in moredetail below.

Referring now to FIGS. 2 and 3, the waveguide 18, made of, for example,copper is brazed in vacuum tight communication to the cylindricalhousing 20, as of copper, of waveguide window structure 19. The housing20 carries transversely therein a section of circular waveguide 21 as ofcopper. A pair of circular, thin dielectric discs 22 are mounted withinsection 21 substantially normal to the direction of propagation ofenergy through section 21, being spaced from each other on either sideof an annular rib portion 23 of section 21. The discs 22 are preferablymade from an alumina type ceramic although any solid dielectric materialwhich is both transparent to electromagnetic waves and capable of beingsealed in vacuum tight communication to the inner wall of section 21 canbe used. For example, A1 0 BeO, fused quartz, single crystal sapphireand boron nitride may be used to advantage. Sealing of the discs 22 tothe inner wall of section 21 is by any of the well known sealingtechniques, as for example, by brazing. The discs 22 together with ribportion 23 form a narrow chamber 24 substantially normal to thedirection of energy propagation.

In a preferred embodiment, the abrupt transitions between rectangular 18and circular 21 waveguides ar made electrically in the order of 11/2wavelengths apart at the center frequency of the passband, where n canbe any integer value with a net capacitive discontinuity existing at thejunction of the waveguides. The discs 22 are centered at the midpointbetween the two transitions, and the thickness of the discs plus thedistance between the discs 22 is subsantially less than one-half anelectrical wavelength at the center frequency of the passband.

The minimum thickness of the discs 22 is determined by mechanicalstrength and ability to maintain a vacuum. Where the discs 22 areparticularly thin the problem becomes more acute. To circumvent thishoops 25 of round cross-section, as of copper wire are brazed to thediscs 22 i and section 21 around the circumferenc of the discs 22.

It has been found that these hoops 25 increase the strength of the discs22 by a factor of 1.5 and affect electrical characteristics onlyslightly. The discs 22 with hoops 25 are less likely to fracture duringbakeout or during high power operation when overheating is experiencedor when there is excessive waveguide pressurization.

A pair of bores 26, 27 in section 21 are aligned with a pair of openings28, 29 in housing 20. A nozzle 30, as of Monel, mounted within opening28 and connected to tubing 31, as of nickel or platinum, mounted withinbore 26 provides an entry-way to chamber 24 from a schematicallyrepresented external fluid circulating means 32, while a similar nOZZle33 and tubing 34 provide an exit Passageway. Of course, additionalentry-ways and exit passageways could be provided.

External fluid circulating means 32 comprises: a fluid circulating pump35 causing a fluid to flow; a heat exchanger 36, for example, a 5kilowatt heat exchanger capable of maintaining the temperature of thefluid between 50-150" F.; and, conduits (not shown) providing connectionbetween the pump 35, chamber 34 and heat exchanger 36.

In the present invention it has been discovered that when the externalfluid circulating means 32 provides a flow, preferably turbulent, of amoderate lossy fluid having a low dielectric constant relative to airthrough chamber 24, the circulation of this fluid at an adequate flowrate serves both to facilitate removal of extra heat and provide coolingof the discs 22, as well as to damp out and prevent excitation of theharmful resonant ghost modes. For example, inert fluorocarbons such asFC- or FC-43 manufactured by the Minnesota Mining and ManufacturingCompany may be used to advantage. These fluids have a dielectricconstant of approximately 1.9 and are some twenty times as lossy as theordinary solid dielectric materials used in waveguide windows. Water,however, which is some twenty three times more lossy than thesefluorocarbons would not be suitable since too much power would bedissipated by the fluid, thus resulting in a substantial reduction inthe power transmitted through the window.

Generally, th selection of the fluid represents a compromise betweenhaving a substance with a loss tangent high enough to damp out andprevent excitation of the harmful resonant ghost modes, yet low enoughto keep the loss of power being transmitted to a minimum. At the sametime, the dielectric constant of the fluid relative to air should be lowenough to minimize impedance matching problems. For higher poweroperation, a useful range of loss tangent would be 0.001 to 0.009, andof dielectric constant relative to air, 1.0 to 3.0. Ideally, the losstangent would be 0.001 and the dielectric constant 1.5. For this casethe discs 22 could be spaced for most efiicient cooling, with lesseffect on the impedance match, and minimum power loss.

In a typical embodiment of the present invention, a waveguide windowstructure to as constructed using AL- 400 ceramic discs 1.4" indiameter, 0.03" thick, spaced 0.08" apart and supported by wire hoops ofcross-sectional diameter 0.062". FC-75 was circulated through thechamber between the discs at the rate of 0.5 gallon per minute. Withtest frequencies centered at 7762 me. (see FIG. 6) the window was ableto transmit kw. of power over a bandwidth of better than 33.3", and witha VSWR under 1.2. No resonant ghost modes were detectable. Approximately1300 watts out of 180 kw., or 0.72% of the input power was dissipated inthe window in the form of heat. The temperature difference across thewindow was 20 C. The embodiment described was found particularlysuitable when transmitting the TE mode, and for damping out all resonantghost modes including TE1111 211 221, sn, TMOIO: and ui- Referring nowto FIGS. 4 and 5 disclosing another embodiment of the present invention,a rectangular half wavelength block 37 of dielectric material as, forexample, alumina ceramic is mounted within a rectangular waveguide 38. Aseries of holes 39 are drilled through block 37 to form fluid ducts.Waveguide 38 also contains apertures 40 which are in alignment withholes 39. The space 41 between housing 42 and waveguide 38 is adapted toreceive a moderately lossy dielectric coolant which flows through nozzle43 in opening 44 through ducts 39 and blocks 37, and out through nozzle45 and opening 46. The fluid is prevented from flowing just through thespace 41 by means of a pair of diametrically opposed septums or fins 47.

Referring now to FIGS. 2 and 7, in sealing the discs 22 to the section21, a metallized coating 48, such as, for example, a brazing alloy isdisposed between disc 22 and section 21 thereby forming a vacuum tightseal. The thickness of the coating 48 about the periphery of disc 22 maybe on the order of 0.005 to 0.020 and sometimes slightly wider.

In practice, a certain amount of the coating material 48 comes up to andbeyond the edge of the disc 22. The coating material flows down the sideof disc 22 to a small extent. Thus a fillet 49 is formed, but with asharp edge 50. Because this sharp edge 50 is in a region of highelectric field, it tends to cause corona discharge and eventuallybreakdown. It has been found that if a member having a curved outersurface is juxtapositioned, as by sealing, on both sides of the disc 22and disposed within the inner walls of the waveguide, as by sealing, andthe radius of curvature of the member is greater than the width of thesealing material, the members will act as corona shields. For mostdevices this would call for the members to have a radius of curvaturegreater than 0.02". Thus hoops 25 serve a second purpose, acting ascorona shields.

While the invention has been described with respect to the output windowof a klystron tube, it is obvious that the invention can be usedanywhere in a microwave system where a window which must transmit highpower over a broad band of frequencies is required.

Since many changes can be made in the above construction and manyapparently widely dilferent embodiments could be made without departingfrom the scope thereof, it is intended that all matter contained in theabove description or shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A microwave transmission device for transmitting electromagneticwaves comprising, conductive wall means defining a waveguide for theelectromagnetic waves, a dielectric structure transparent to said wavessealed across said waveguide, said dielectric structure having a fluidpassageway therethrough positioned in energy exchanging relationshipwith said waves passing therethrough, means forming a dielectric fluidhaving a loss tangent between 0.001 and 0.009 inclusive, and means forcirculating said dielectric fluid through said passageway for modedampening said dielectric structure.

2. The device according to claim 1 wherein said fluid has a dielectricconstant relative to air between 1.0 and 3.0 inclusive.

3. The apparatus of claim 1 wherein said dielectric structuretransparent to said waves includes, a pair of spaced apart soliddielectric members transparent to said waves and sealed across saidwaveguide, and wherein said fluid passageway through said dielectricstructure includes a fluid chamber defined between said dielectricmembers and by said waveguide.

4. The apparatus of claim 1 wherein said dielectric structure includes,a block of dielectric material transparent to said waves sealed acrosssaid waveguide, said block containing a plurality of spaced boresforming said fluid passageway.

References Cited UNITED STATES PATENTS 2,925,515 2/1960 Peter 31539.3 X2,958,834 11/1960 Symons et a1 33398 2,990,526 6/1961 Shelton 333-983,101,461 8/1963 Henry-Bezy et a1 33398 3,110,000 11/1963 Churchill33373 3,158,823 11/1964 Bird et al 17415 X FOREIGN PATENTS 669,250 4/1952 Great Britain.

ELI LIEBERMAN, Primary Examiner.

MARVIN NUSSBAUM, Assistant Examiner.

